FIG. 16 is a circuit diagram of conventional acceleration sensor 501 described in Patent Document 1. Acceleration sensor 501 includes detector element 1 for changing an output according to acceleration, driver circuit 2 for inputting an AC bias voltage signal to detector element 1, and detector circuit 3 for outputting an acceleration signal based on the output of detector element 1. Detector element 1 includes fixed electrodes 4A and 4B, and movable electrodes 5A and 5B disposed between these fixed electrodes 4A and 4B. Respective one ends of movable electrodes 5A and 5B are connected to driver circuit 2.
Acceleration applied to detector element 1 of acceleration sensor 501 causes movable electrodes 5A and 5B to swing like a pendulum with the one ends as fulcrum, hence changing a distance between fixed electrode 4A and movable electrode 5A and a distance between fixed electrode 4B and movable electrode 5B. The changes of the distances change a capacitance generated between movable electrode 5A and fixed electrode 4A and a capacitance generated between movable electrode 5B and fixed electrode 4B. Detector circuit 3 then detects the acceleration based on these changes in the capacitances.
Detector circuit 3 includes operational amplifiers 6 and 7, differential amplifier 8 for amplifying a difference between outputs of operational amplifiers 6 and 7, and defect detector 12 connected to an output port of differential amplifier 8. Operational amplifier 6 has input port 6A receiving an output of fixed electrode 4A, and input port 6B selectively connected to reference voltage generator 9A and variable voltage generator 10A. Operational amplifier 7 has input port 7A receiving an output of fixed electrode 4B, and second input port 7B selectively connected to reference voltage generator 9B and variable voltage generator 10B. Resistor 11A is connected between input port 6A and output port 6C of operational amplifier 6. Resistor 11B is connected between input port 7A and output port 7C of operational amplifier 7.
While no acceleration is applied to detector element 1, movable voltage generators 10A and 10B are connected to input ports 6B and 7B, respectively, and respective voltages to variable voltage generators 10A and 10B are determined to be predetermined voltages so as to apply acceleration virtually to detector element 1 to detect a defect in detector element 1.
More specifically, if the voltages of variable voltage generators 10A and 10B are determined to be 1.5V and 3.5V, respectively, movable electrode 5B moves towards fixed electrode 4B, and movable electrode 5A moves away from fixed electrode 4A. This movement changes a capacitance produced between fixed electrode 4A and movable electrode 5A, and a capacitance produced between fixed electrode 4B and movable electrode 5B, hence generating currents. These currents run through resistors 11A and 11B, and generate voltages at resistors 11A and 11B. These voltages are input to differential amplifier 8, and an output signal from differential amplifier 8 is sent to defect detector 12.
Defect detector 12 determines that there is no defect if the output signal of differential amplifier 8 is within a predetermined voltage range. If the output signal is out of the predetermined voltage range, defect detector 12 determines that there is a defect. For example, if fixed electrode 4A is defective, no current is generated from fixed electrode 4A, and thus no voltage is generated in resistor 11A. Accordingly, the output signal from differential amplifier 8 exceeds the predetermined voltage range, and therefore, defect detector 12 determines that there is a defect in detector element 1.
Conventional acceleration sensor 501 requires at least two variable voltage generators 10A and 10B for detecting a defect, hence preventing acceleration sensor 501 from having a small size.    Patent Document 1: JP2007-248327A